This invention relates to a subframe mount structure which utilizes a crashproof detachable mechanism in a joint portion (or mount) where the subframe for a vehicle is mounted in a vehicle body, to improve a passenger protection capability of the vehicle.
Various techniques have been proposed to increase a crashable stroke so as to obtain improved energy absorption of crashing loads applied on collision of the vehicle, thereby preventing a vehicle's interior from deforming due to rearward displacement of a front frame of the subframe caused by impact of the collision.
Among such techniques is a subframe-fastening structure for a vehicle in which a bolted joint portion between a subframe for supporting a suspension arm and a vehicle body is ruptured to separate the subframe from the vehicle body, which is disclosed, for example, in Japanese Laid-Open Utility Model Application, Publication No. 52-11717.
Several means for absorbing energy of crashing loads applied on collision of a vehicle are known in the art, and include “sliding separable structure” having a subframe allowed to slide across a vehicle body when a collision occurs, and “breaking separable structure” having a subframe allowed to be partially broken so as to separate the subframe from the vehicle body. Two varieties of subframe mount structures are available to which the “sliding separable structure” and “breaking separable structure” may be applied: one is the four-point support system in which four joint points (spots) of a subframe are supported on a vehicle body and two joint points (spots) thereof are to be separated on collision; and the other is the six-point support system in which six joint points (spots) of a subframe are supported on a vehicle body and two joint points (spots) thereof are to be separated on collision.
Bolts and nuts used in the subframe-fastening structure for a vehicle as disclosed in the above publication, however, have a floating support structure using a rubber vibration isolator, which allows the subframe to float on the vehicle body, for the purpose of improving marketability of the vehicle in view of noise and vibration insulation. From a double-safety standpoint, i.e., for preventing an engine of the vehicle from falling off when collision occurs, the subframe-fastening structure using bolts and nuts is designed to prevent a fall of the engine, which would otherwise result from rupture of the rubber vibration isolator caused by crash associated with collision of the vehicle. This engine falling prevention functionality would disadvantageously inhibit separation of the vehicle body and the subframe, and mar a desirable failure mode according to which the vehicle body should be destroyed on collision.
When a relatively large subframe, e.g., a subframe for a double wishbone suspension, is supported with a subframe mount structure having the aforementioned six-point support system where six joint points of the subframe are supported on the vehicle body, in order to increase a crashable stroke on collision, many joint points, particularly, more than two joint points except for two fundamental joint points, of the subframe should be ruptured stably without fail to allow the subframe to separate from the vehicle body.
When many joint points of the subframe at which the subframe is mounted on the vehicle body are ruptured so that the subframe is separated from the vehicle body, the separation caused by collision of the vehicle progresses stepwise, i.e., several incidents of separation take place at intervals, thus intermittently producing an action tending to stop an inertial force of the collision. Accordingly, each time when the joint point of the subframe is ruptured to allow the subframe to be separated from the vehicle body, a force that stops the inertial force is transmitted directly to a passenger and acts as an impact on the passenger. Therefore, with consideration given to an adverse effect of the impacts on the passenger, it is desired that each joint point be ruptured with a relatively stable (unvarying) load.
The present invention is made to address the above-described disadvantages in the existing techniques, and it is an exemplary object of the present invention to provide a subframe mount structure that can mitigate damage of a passenger caused on collision of the vehicle to protect the passenger.